Generally, phosphors are opaque polycrystals having particle diameters of several micrometers to several hundreds of micrometers. They can be obtained by doping, for example, rare earth ions as an activator into a parent material such as silicate, aluminate, oxysulfide, etc. Major applications of phosphors are fluorescent lamps and various displays. Phosphors also are used for fluorescent display tubes, intensifying screens for radiography, fluorescent screens for radiography, markings for preventing forgery, solid-state lasers, etc. Recently, fine phosphor particles with submicron to nano sizes also are synthesized (see Toshiyuki MASUI, Nobuhito IMANAKA, “Recent Advance in Rare Earth Phosphor Fine Particle”, Materials Integration, 2004, vol. 17, No. 3, pp. 10-14). So far, phosphors have been made finer for the purpose of improving the characteristics of existing devices, which is typified by an increase in screen display definition of displays, for example.
Furthermore, recently, upconversion emission in which red, blue, and green visible lights are generated by infrared irradiation is studied with various phosphors (Taiju TSUBOI, “Multi-Photon Excited Luminescence by Rare-Earth Ions”, Materials Integration, 2004, vol. 17, No. 3, pp. 36-44).
WO99/37836 (JP2002-501126A) discloses light-emitting fibers in which inorganic light-emitting pigments are dispersed.
JP11(1999)-288785A discloses an optically functional thin film that has a porous layer composed mainly of Si on a substrate and a structure in which fine phosphor particles are dispersed in the porous layer.
JP2002-105409A discloses a coating composition containing an organic binder as well as fibers that contain a fiber-forming material and a light-emitting dye or light-emitting pigment dispersed therein.
JP2004-292599A discloses upconversion fine phosphor particles characterized in that the general formula thereof is expressed by a composition formula of (R1-x, Erx)2O3 (where R denotes at least one of Y, La, Gd, and Lu, while X denotes the molar amount and satisfies the range of 0.001≦X≦0.20), and they are excited by light with a wavelength in the range of 500 nm to 2000 nm and thereby undergo upconversion emission.